Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 PM EDT Tue Mar 28 2023 Valid 12Z Fri Mar 31 2023 - 12Z Tue Apr 04 2023 ***Strong low pressure to produce some severe thunderstorms, episodes of heavy rainfall, and windy conditions across portions of the Plains and Midwest late week into the weekend*** ...Synoptic Overview... The main weather story will be the strong low pressure system developing across the Central Plains and the Midwest to close out the work week. As the upper trough from the western U.S. ejects over the western High Plains, lee cyclogenesis will commence over eastern Colorado Thursday night and a well organized low pressure system will evolve over the Central Plains along a rather strong frontal boundary. Severe storms, heavy rainfall, and gusty winds will be making weather headlines going into Friday and persisting into early Saturday. This system will then move in a general northeastward direction through the Great Lakes by Saturday and then to southeastern Canada by Sunday. Meanwhile, energy ahead of the next upper level trough/closed low dropping down from the Gulf of Alaska should reach the Pacific Northwest by Saturday and then the larger scale trough will likely settle over the West by early next week. This system will bring another cold front across much of the western U.S. to reinforce the cool weather pattern and support multiple days of rain and potentially heavy mountain snow from the Pacific Northwest and northern California into the northern-central Rockies. This latter western system may produce another surface low developing over the central/southern Plains early next week, with strength and position determined by depth and timing of the supporting dynamics. ...Model Guidance/Predictability Assessment... The updated forecast started with a composite of 06Z/00Z operational models for late this week into the weekend and then trended toward a late-period even mix of operational models and 06Z GEFS/00Z ECMWF means, with the operational components split among the 00Z and 12Z/27 ECMWF runs along with switching GFS input more to the 00Z run based in light of differences/guidance comparisons. Over the past day the guidance has trended somewhat closer together for the storm tracking through the Great Lakes by Saturday, with latest consensus holding onto a closed or nearly closed upper low as seen in recent GFS runs but with timing somewhat faster than earlier GFS runs but a little slower than the ECMWF. The 12Z CMC is now on the slower side of this majority cluster. Behind this system, guidance has held onto using leading Pacific energy tracking along the U.S.-Canadian border to support a surface system reaching near Lake Superior by next Monday. Most GFS runs and the 12Z CMC are on the slower side of the spread while ECMWF runs have tended to be on the fast side, with an intermediate/ensemble mean solution holding steady. Developing differences with upstream Pacific/Alaska flow late in the period start to influence progression of the upper trough moving into the West early next week. Latest GFS/ECMWF runs have shown quite a bit of waffling for specifics, with the 00Z GFS and new 12Z GFS comparing somewhat better to the means than the flatter/faster 06Z run, and an average of the past two ECMWF runs looking more appealing than either one on its own. Thus a multi-model-run/ensemble blend provided the best large scale representation of the western trough aloft late in the period. ...Weather/Hazards Highlights... The most impactful weather during this forecast period is expected on Friday in association with the strengthening low pressure system across the Plains and Midwest, and March will not be going out like a lamb here. Perhaps the greatest concern will be the elevated threat of severe thunderstorms in the broad warm sector extending from the greater Chicago metro area to the ArkLaTex region based on the latest Storm Prediction Center outlook. A combination of both strong kinematics and instability parameters will create a favorable environment of severe storms with increasing wind shear and thunderstorm updraft potential, and this is something that will need to be closely monitored in the days ahead as the event enters the short range. In addition to the intense convection, widespread windy conditions are likely to develop across a broad expanse of the Plains and Midwest as the surface pressure gradient tightens, with gusts in excess of 40 mph expected behind the cold front. There has also been a trend for a band of moderate to locally heavy snow from southern South Dakota to southern Minnesota to the northern Great Lakes, along the expected deformation band northwest of the main surface low. The potential exists for some 6-12 inch totals across parts of this region. Swaths of heavy rainfall are also likely farther south from the Ohio Valley to the Deep South, but the relatively progressive nature of this storm system should keep rainfall totals from becoming extreme. Fairly intense rain rates for a short duration of time could still lead to localized issues though. These considerations and lack of strong guidance clustering for location(s) of highest rainfall totals merit only a Marginal Risk of excessive rainfall in the Day 4 Excessive Rainfall Outlook (12Z Friday-12Z Saturday), though will continue to monitor any emerging trends that could help to resolve any embedded Slight Risk area. There is a lingering Marginal Risk area near the Gulf Coast on Day 5 (12Z Saturday-12Z Sunday) for what is left of the Day 4 activity over the Southeast plus possible localized maxima farther west, as the front settles along/just south of the Gulf Coast. Elsewhere across the nation, reloading of upper level troughing over the northwestern U.S. by the weekend and continuing into early next week will support multiple days of rain/mountain snow with terrain enhancement from the Pacific Northwest and northern California into the northern and central Rockies, with the moisture shield pushing gradually farther southeast with time. Heavy snow is likely for the Cascades and the Olympic Mountains where a few feet of snow accumulation is possible at higher elevations. Significant snow may reach the Sierra Nevada by early next week. Showers and thunderstorms may return to locations from the Deep South to the Ohio Valley again on Tuesday as the next storm system gets better organized across the central Plains. Much of the western U.S. will continue to have cooler than normal temperatures for most of this forecast period as the next upper level trough becomes anchored over the region. The best potential for highs 15 to 25 degrees below normal will be Friday across the Dakotas as the first upper trough passes through, and then over a broader expanse from the West Coast to the Dakotas early next week as the second trough settles in. Scattered record cold highs may be possible. Upper level ridging extending from a Gulf of Mexico upper high should encourage warm and humid conditions rebuilding across the south-central U.S. to spread into the Mississippi Valley Friday and the East by Saturday, with highs generally 5-15F above average and lows around 15-25F warmer than normal. After a very brief cooling trend, above normal temperatures should again spread from the central U.S. on Sunday into the rest of the East by Monday-Tuesday. A few record warm low temperatures could be tied or set, and next Tuesday may offer the possibility for some record highs over the South. Rausch/Hamrick Additional 3-7 Day Hazard information can be found on the WPC medium range hazards outlook chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation, experimental excessive rainfall outlook, winter weather outlook probabilities and heat indices are at: https://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif https://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif https://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html https://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml https://www.wpc.ncep.noaa.gov/#page=ero https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml